Mast Cells play an important role in many inflammatory and immunological reactions by releasing an array of mediators. The goal of our studies is to understand the intracellular signal transduction pathways that lead to the release of these molecules. In previous studies we observed that protein tyrosine phosphorylation is an early and critical signal for FceRI induced degranulation. The protein tyrosine kinase Syk was found to be tyrosine phosphorylated and activated after receptor aggregation. A variant of the RBL-2H3 mast cells that has no detectable Syk was also identified and demonstrated that Syk is essential for the receptor-induced release of inflammatory mediators. Analysis of signaling pathways in these cells indicate that most of the FceRI-induced cellular protein tyrosine phosphorylation is downstream of Syk and requires enzymatically active Syk. The Syk negative mast cells were used to examine the role of Syk in immune receptor induced activation of cells. CD45 was essential for ZAP-70, but not for Syk, to reconstitute antigen receptor-initiated degranulation signals in mast cells. The SH2 domain-mediated membrane translocation of Syk was essential for immune receptor mediated activation of Syk for downstream signaling events and the localization of Syk in glycosphingolipid-enriched microdomains by itself was not enough to generate or enhance signaling events. Syk was found to regulate molecules including the protein tyrosine kinase Btk and the guanine nucleotide exchange factor Vav. Both of these proteins together with the protein tyrosine phosphatase SHP-1 regulate the c-Jun N-terminal kinase (JNK) pathway that leads to transcriptional activation of cytokine genes.